The present invention provides a hafnium metallocene catalyst useful in the polymerization of olefins, particularly propylene and higher alpha-olefins, and in the copolymerization of olefins, especially ethylene and propylene.
The use of metallocenes as catalysts for the polymerization of ethylene is known in the art. German patent application No. 2,608,863 discloses a catalyst system for the polymerization of ethylene consisting of bis(cyclopentadienyl)titanium dialkyl, an aluminum trialkyl and water. German patent application No. 2,608,933 discloses an ethylene polymerization catalyst system consisting of zirconium metallocenes of the general formula (cyclopentadienyl).sub.n Zr Y.sub.4-n, wherein Y represents R.sub.1 CH.sub.2 AlR.sub.2, CH.sub.2 CH.sub.2 AlR.sub.2 and CH.sub.2 CH (AlR.sub.2).sub.2 where R stands for an alkyl or metallo alkyl, and n is a number within the range 1-4; and the metallocene catalyst is used in combination with an aluminum trialkyl cocatalyst and water.
The use of metallocenes as catalysts in the copolymerization of ethylene and other alpha-olefins is also known in the art. U.S. Pat. No. 4,542,199 to Kaminsky, et al. discloses a process for the polymerization of olefins and particularly for the preparation of polyethylene and copolymers of polyethylene and other alpha-olefins. The disclosed catalyst system includes a catalyst of the formula (cyclopentadienyl).sub.2 MeRHal in which R is a halogen, a cyclopentadienyl or a C.sub.1 -C.sub.6 alkyl radical, Me is a transition metal, in particular zirconium, and Hal is a halogen, in particular chlorine. The catalyst system also includes an alumoxane having the general formula Al.sub.2 OR.sub.4 (Al(R)-O).sub.n for a linear molecule and/or (Al(R)-O).sub.n+2 for a cyclic molecule in which n is a number from 4-20 and R is a methyl or ethyl radical. A similar catalyst system is disclosed in U.S. Pat. No. 4,404,344.
U.S. Pat. No. 4,530,914 discloses a catalyst system for the polymerization of ethylene to polyethylene having a broad molecular weight distribution and especially a bimodal or multimodal molecular weight distribution. The catalyst system is comprised of at least two different metallocenes and an alumoxane. The patent discloses metallocenes that may have a bridge between two cyclopentadienyl rings with the bridge serving to make those rings stereorigid.
European Patent Publication No. 0185918 discloses a stereorigid, chiral zirconium metallocene catalyst for the polymerization of olefins. The application does not indicate that hafnium could be substituted for the zirconium and used to produce a useful polymer product. The bridge between the cyclopentadienyl groups is disclosed as being a linear hydrocarbon with 1-4 carbon atoms or a cyclical hydrocarbon with 3-6 carbon atoms.
It is known by those skilled in the art that polyolefins, and principally polypropylene, may be produced in various forms: isotactic, syndiotactic, atactic and isotactic-atactic stereoblock. Isotactic polypropylene contains principally repeating units with identical configurations and only a few erratic, brief inversions in the chain. Isotactic polypropylene may be structurally represented as ##STR1## Isotactic polypropylene is capable of forming a highly crystalline polymer with crystalline melting points and other desirable physical properties that are considerably different from the same polymer in an amorphous, or noncrystalline, state.
A syndiotactic polymer contains principally units of exactly alternating configurations and is represented by the structure ##STR2## A polymer chain showing no regular order of repeating unit configurations is an atactic polymer. In commercial applications, a certain percentage of atactic polymer is typically produced with the isotactic form. A polymer with recurring units of opposite configuration is an isotactic stereoblock polymer and is represented by ##STR3## This latter type, the stereoblock polymer, has been successfully produced with metallocene catalysts as described in U.S. Pat. No. 4,522,982.
A system for the production of isotactic polypropylene using a titanium or zirconium metallocene catalyst and an alumoxane cocatalyst is described in "Mechanisms of Stereochemical Control in Propylene Polymerization with Soluble Group 4B Metallocene/Methylalumoxane Catalysts," J. Am. Chem. Soc., Vol. 106, pp. 6355-64, 1984. The article shows that chiral catalysts derived from the racemic enantiomers of ethylene-bridged indenyl derivatives form isotactic polypropylene by the conventional structure predicted by an enantiomorphic-site stereochemical control model. The meso achiral form of the ethylene-bridged titanium indenyl diastereomers and achiral zirconocene derivatives, however, produce polypropylene with a purely atactic structure.
The above references generally describe metallocene catalysts as having two cyclopentadienyl rings attached to a transition metal. This general description would include a hafnium metallocene, but prior to the present invention, the references have disclosed only titanium and zirconium metallocene catalysts that prove to be of any use. Heretofore, hafnium metallocene catalysts have not produced useful polypropylene products, in particular, as the molecular weights of the products have been well below 10,000. Hafnium could not simply be substituted for zirconium or titanium in known metallocene systems and produce useful results.
The present invention includes the discovery of a new type of hafnium metallocene catalyst not taught by the prior art. The catalyst exhibits properties and produces results that surpass those exhibited by zirconium and titanium metallocene catalysts with similar structures and used under similar conditions.